Quotations

Quotations are the general term for anonymous functions or lambdas. They denote a snippet of code which can be executed later. Their exact syntax varies by language. For example, Factor denotes quotations using square-brackets. Whereas, Tal and Postscript denote them using curly-brackets. Regardless of syntax, they are core to constructing higher-order functions in concatenative languages.

Higher-Order Function

A higher-order function is an operation that utilize other functions as inputs or create new functions as outputs. This feature is leverage by a core utility of concatenative programming: combinators. They enable a programmer to name code not values.

Example of Quotations

Factor

! { 1 4 9 16 }
{ 1 2 3 4 } [ sq ] map 

Kitten

// [10, 20, 30]
[1, 2, 3] { (* 10) } map

Tal

(02 04 06 08 0a)
{ 01 02 03 04 05 } STH2r { STH2k LDAk DUP ADD STH2r STA JMP2r } STH2r foreach

Implementation of foreach:

@foreach ( {bytes}* fn* -- bytes* )
	,&t STR2
	DUP2k #0002 SUB2 LDA2 ADD2 SWP2
	&l ( -- )
		[ LIT2 &t $2 ] JSR2 INC2 GTH2k ?&l
	POP2 POP2 JMP2r